1. Field of the Invention
The invention relates to mobile communications networks, cell neighbor determination and a method for automatic neighbor cell relation reporting in a mobile communication system.
2. Description of the Related Art
Radio network planning is an important part in the building and maintaining of a cellular mobile communication network. In radio network planning it is determined, for example, the placing of base stations, the required dimensioning of base station transmitters and receivers, and base station parameterization governing transmission power or frequency allocation. There are sophisticated tools for automatically determining the placing of base stations in the area of a network. It is possible in these tools to define closely certain elementary characteristics of the network area such the terrain and buildings in the area. An important part of the radio network planning has been the determination of neighboring cells for each cell in the network. An important objective of the radio network planning tools has been the determination of neighboring cells. The correct information about neighboring cells is important for the performing of high-quality handovers between cells and the avoiding repeating frequent handovers. The handover must not be established to random cells that happen to provide an acceptable radio quality only at short random time interval. The problem is made worse by multi-path propagation of radio waves. The handover must also not be performed to cells that belong to a different network with which there may not be a roaming agreement or to cells for which handovers are not allowed for traffic reasons. Therefore, mobile nodes may not request handovers to target cells on their own.
The actual radio conditions may differ from the radio conditions determined using radio network planning tools. The cell sizes are becoming increasingly smaller. Nowadays, the cell beams are no longer omnidirectional, but instead cells with narrower beams are used. The transmission power of a cell may also vary. These are also factors that make the defining of neighboring cells more difficult to perform beforehand.
Due to the aforementioned factors it would be important to obtain automatically information about neighboring cells to a UMTS radio network.
Recently, 3G Partnership Project (3GPP) has defined an initiative for defining an Automatic Neighbor Relation (ANR) function in order to relieve the operator at least partly from the burden of manually managing Neighbor Relations (NRs). The ANR function builds a Neighbor Relation Table (NRT) to a Radio Network Controller (RNC) based on reports from the mobile node. The ANR function adds to a predefined NRT information on neighbor cells missing from the NRT. The problem with the reporting is that continuous reports from mobile nodes whenever they encounter new cells may cause a significant message load to an RNC and a NodeB. Therefore, there is a need to limit the messaging required for maintaining the NRTs in the RNCs. The need for reporting may also vary depending on whether a cell is located in a newly introduced part of the radio network or whether the cell is located in a relatively static and established part of the radio network. In densely populated urban areas, it is important to rapidly report any changes in the radio network cell neighbor relations. Further, the immediate reporting of every new cell when encountered drains mobile node battery especially during idle time, due to the superfluous messaging with a NodeB. Therefore, it would be beneficial to control how often neighbor relations are reported by a mobile node or generally by all mobile nodes within a given cell.